A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus generally includes a support structure to hold an object, e.g. the substrate or the patterning device, wherein the support structure is moveable in a direction relative to a reference structure, e.g. a metrology frame.
The position of the support structure relative to the reference structure can be measured by a position measurement system. However, most position measurement systems have favorable properties in a certain frequency range only, because some position measurement systems are dependent on environmental conditions like pressure and temperature, which are low-frequent phenomena, giving a good position measurement only in a high-frequency range, and other measurement systems are easily disturbed by dynamical behavior like resonances, and therefore only provide accurate measurements in a low-frequency range. The favorable frequency range may limit controller capabilities of a corresponding positioning system, which may be further limited by measured resonances, resulting in a compromised positioning accuracy of the support structure relative to the reference structure.
An example of a position measurement system is an encoder type measurement system including a grating and a sensor head, also referred to as encoder, cooperating with the grating. The grating is usually provided on a plate and attached to the support structure or the reference structure. The sensor head is then provided on the other one of the support structure and reference structure. However, the plate may, due to its design and dimensions, be a relatively non-stiff member sensitive to high-frequency dynamical behavior. Due to this property, the encoder measurement system has a favorable low-frequency behavior. For higher frequencies, the measurement signal no longer represents the real position of the support structure or an object held by the support structure.
When the plates have a dynamical behavior depending on a measurement location on the plates, it may be difficult to find a proper controller that works in all locations.
The favorable frequency range of the position measurement system may also be effected by the measurement location. For example, if the sensor head and plate are not sensitive to dynamic behavior, then the frequency range may still be limited by the dynamics of the part of the support structure to which the sensor head or plate is attached to.